In nuclear reactor systems, a number of the components, such as nuclear fuel rods, burnable absorber rods and control rods, are formed of metallic tubes or cladding. These metallic tubes contain solid materials such as nuclear fuel, a burnable absorber, and neutron absorbers, within a helium atmosphere in the hermetically sealed cladding, which solid materials form tritium during operation of the reactor. Such tritium is formed during reactor operation by ternary fissioning of uranium in fuel rods, or by nuclear reactions with boron in the burnable absorber rods or control rods.
Although these rods are hermetically sealed, tritium can diffuse through the cladding at reactor operating conditions, especially through a stainless steel cladding, and can pose a health hazard. It is thus desirable to contain the tritium hermetically sealed within the fuel rod, burnable absorber rod or control rod.
In U.S. Pat. No. 4,100,020 there is described a fuel pin which contains a solid oxidizing chemical, such as oxides of a transition metal, which will radiolytically and thermally decompose to release oxidizing chemicals or free oxygen to the interior of the pin so as to form zirconium dioxide on the inner walls of the zirconium alloy from which the pin is made. Such solid materials may be added as separate pellets, in addition to the fuel pellets in the pin, or as an oxidizing chemical in the form of a coating on the inner wall of the pin. The purpose of the addition of the solid oxidizing chemical is to form an oxide coating on the cladding to replace failure of an initial oxide coating thereon by abrasion or the like, to prevent attack by fission products which could cause stress corrosion cracking and possible perforation of the cladding.